Perhaps it should be said that optimal nutrition is an ultimate goal which science is not yet prepared to define descriptively in detail. Speaking operationally, we may say that recent research has established, fully and objectively, the principle of the nutritional improvability of the normal. The experimental evidence can, of course, be but sketchily presented in a review of this sort which attempts to summarise in so little space a scientific advance of undoubtedly far‐reaching significance. Under the necessity of extreme brevity, the writer trusts he will be pardoned for drawing illustrations chiefly from the work with which he is best acquainted. In experiments to determine what proportion of protective food suffices to balance a minimum proportion of wheat in the diet, it was found that a mixture of five‐sixths ground whole wheat and one‐sixth dried whole milk with table salt and distilled water (Diet A) was adequate in that it supported normal growth and health with successful reproduction and rearing of young, generation after generation. Yet when the proportion of milk was increased (Diet B) the average results were better. In the experiments just mentioned, an already‐adequate dietary and an already‐normal condition of nutritional wellbeing and health were improved by a more scientific adjustment of the relative quantities in which the staple articles of food were consumed. And in the comparison of the effects of these two diets the principle of the nutritional improvability of the normal was manifested measurably at every stage of the life cycle. Growth and development, adult vitality, and length of life all were normal on Diet A and all were better on Diet B. This research having been planned in terms of natural articles of food, the sole experimental variable was the quantitative proportion or ratio between the foods constituting the dietary. If, on the other hand, we turn to the consideration of individual chemical factors, we find that the single change in proportions of staple foods had the effect of enriching the dietary at four points: protein, calcium, riboflavin, and vitamin A. Subsequent experimentation was planned both in terms of these four chemical factors separately and in terms of diversification of the dietary by addition of natural foods of other types. Here it was found that enrichment of the original diet with protein alone or its diversification with other natural foods tended to a moderate increase in growth and adult size, but no distinct improvement in the life history. Clearly this indicates that the increased intake of protein played but little if any part in the nutritional improvement induced by Diet B over Diet A; and also strengthens the probability that the observed improvement is essentially explainable in terms of the factors we recognise, for if anything unknown had played an important rôle in this improvement, the diversification of the diet would probably have revealed some indication of it. Calcium, riboflavin, and vitamin A each is found to play a signicant part in the nutritional improvement of the already adequate diet and already normal health. With each of these three factors the level of intake giving best results in long‐term experiments is two or more times higher than the level of minimal adequacy. Some aspects of the respective rôles of these three factors are still subjects of further experimental investigation. It is not to be assumed that the wide margins of beneficial intake over actual need, found as just mentioned with calcium, riboflavin, and vitamin A, will apply to the other nutritionally essential mineral elements and vitamins. Each should be investigated independently in this respect; and with no presuppositions derived from the findings with calcium, riboflavin, and vitamin A, for these were not random samples, but were taken for rigorous experimental study because of the definite suggestions of earlier work. Meanwhile the above‐mentioned findings with the factors already comprehensively investigated afford a basis both for clarification of a fundamental chemical principle in nutrition, and for its practical application. One useful first‐approximation of nineteenth‐century science was that an organism may be expected to grow only as fast or as far as is consistent with the specific chemical composition of its kind; and another was that it is the fixité of the organism's internal environment which enables it to cope with new or changing external environments. It is surprising that these views continued to be held so rigidly for so long when at the same time there were developing physico‐chemical principles which call for a more flexible concept. In this light it seems clear that the so‐called steady states of the body are only relatively so: that one cannot introduce into the system different amounts and proportions of such active factors as we know some food constituents to be, without some resulting changes of concentration levels or of dynamic‐equilibrium points, or both. And now we have the objective evidence of well‐controlled, long‐term experimentation showing nutritional improvement of an already normal bodily condition in such manner as seems best expressed by saying that the chemical aspect of the body's internal environment has been modified for the better. Thus in accordance with physico‐chemical principles we now conceive the “normal level” of each nutritional factor to be not a single fixed level but a zone. Undoubtedly this zone is wider for some factors than for others, and probably also the most advantageous level is with some substances near the upper margins, and with other substances near the middle or the lower margins, of the respective normal zones. Thus while our bodies enjoy by virtue of their biological inheritance certain self‐regulatory processes of striking effectiveness, our minds are now finding, through chemical research, how these can be made still more effective by the scientific guidance of our nutritional intakes; by helpfully influencing our internal environments through good habits in our daily choice of food. Contemporary research in the chemistry of nutrition is here developing a fundamental and far‐reaching scientific concept which hitherto has hardly been apprehended because species have been regarded as more rigidly specific in their chemical composition, and the “steady states” of their internal environment have been regarded as more rigidly fixed, than they really are. The accepted generalisation that each life history is determined (1) by heredity and (2) by environment assigns all except hereditary factors to environment by definition. But as the result of nearly a century of scientific as well as popular habit of thought, the word “environment” actually connotes surroundings. Science exaggerated the extent and rigidity with which our internal chemistry is automatically regulated by our biological inheritance, to such an extent that there seemed nothing for us to do about it except to admire its wonders and stand ready to repair its occasional breakdowns. But now that we are finding ways to add conscious chemical control and improvement to the marvellous mechanism with which nature endows us, we can be not merely repair‐men to a biologically inherited bodily machine, but also architects of a higher health. It may help to make this newly‐opened opportunity clearer if, instead of the above‐mentioned two, we think and speak of three major determinants of our life‐histories: (1) heredity; (2) environment, in the familiar external sense of surroundings; and (3) the body's internal environment, which immediately environs and conditions the life process, and which in the course of the life cycle is much more significantly influenced than hitherto supposed by even the normal differences in what we take into our bodies as food. This responsiveness of our internal chemistry, and resulting degree or level of positive health, to our nutritional intake, usually becomes manifestly measurable only in cases of visible injury from nutritional deficiency, which, once apprehended, we seek to avoid; or in experimentation with laboratory animals whose natural life‐cycles are such as to permit of accurately controlled conditions and observations extending throughout entire lifetimes and successive generations. In the long‐controlled, laboratory‐bred colony of experimental animals used in large numbers for full‐life and successive‐generation feeding tests conducted with all the quantitatively meticulous care and precautions to which research workers in the exact sciences are trained, we now have an instrument and technique of investigation such as has not existed before. Much remains to be done in the new field of research thus opened; but work already completed shows clearly the possibility of nutritional improvements of already‐normal health, vitality and efficiency throughout our lives. Whatever we are individually born with, we can each do more for ourselves to influence our life histories in the direction of our aspirations than science has hitherto thought.
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